The overall objectives of this investigation are to test the clinical efficacy of stimulant treatment in children with Conduct disorders and to provide detailed objective descriptions of their school behavior. The study has been designed 1) to investigate the effect of methylphenidate compared with a placebo in children, 6 to 13 years of age, with Conduct disorders; and 2) to evaluate the school behavior of children with Conduct disorders. Inclusion and exclusion diagnostic are stipulated. Children with "pure" Conduct disorders will be randomly assigned to methylphenidate or placebo for 5 weeks in a fixed/flexible dosage regimen (2 weeks fixed, 3 weeks flexible). Children's behavior will be recorded by trained observers in structured and unstructured school settings. Data will be examined to determine whether different patterns of classroom behavior occur in children with conduct disorders and normal children, in supervised as well as unsupervised school settings.